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Atomic-Scale Modeling of PEO-Based Solid Polymer Electrolytes for Solid-State Rechargeable BatteriesAuthor: Cheerla Ramesh Date: 2017-05-23 Report no: IIIT/TH/2017/29 Advisor:Marimuthu Krishnan AbstractBatteries are electrochemical devices that convert chemical energy into electrical energy. An electrochemical cell of a battery composed of three major components: an anode, a cathode and an electrolyte. The electrolyte is an ionic conductor that provides a medium for transfer of ions between the electrodes of a cell. The amount of energy stored in a battery depends critically on the nature of electrochemical reactions at the electrode-electrolyte interfaces and the choice of the electrolyte. Solid polymer electrolytes (SPEs) consisting of alkali salts dissolved in a solid polymer matrix of poly(ethylene oxide) (PEO) are novel ion transporters that offer several advantages over other conventional electrolytes for use in all-solid-state rechargeable batteries. In order to fully exploit the advantages of SPEs and for rational design of novel SPEs with higher conductivity and higher energy density for future applications, it is necessary to understand the molecular-level details of the mechanism and the critical factors governing the ion conduction in these materials. Direct observation of migration pathways of ions and a quantitative dissection of their en- ergetics in solid polymer electrolytes (SPEs) are essential to understand the molecular origins of barriers limiting the conductivity of these novel materials. Depending upon the interplay between molecular packing and dynamics, SPEs exhibit a wide range of conductivity (10−9 - 10−4 S/cm) at room temperature despite their common polymer matrix. The principal driving forces controlling the degree of conductivity of SPEs including the nature of macromolecular packing, ion-polymer interactions, ion coordination structure, conformational order and mo- bility of polymers are significantly different among the SPEs. However, the precise correla- tions between these molecular factors and the ion transport in SPEs are not well established. It remains unclear whether disordered amorphous or ordered crystalline structures promote conduction of ions in SPEs. The other important challenges that remain in SPE-related re- search are the identification of low-energy ion conduction pathways, characterization of the free energy profiles and the structural and dynamical contributions to the favorable (stable energy wells) and unfavorable (activation energy barriers) ion sites along these conduction pathways. ix Detailed molecular studies are needed to establish a precise correlation between the na- ture of polymer packing, dynamics, energetics, and ion conduction for rational design of SPE-based fast ion conductors. In this regard, molecular dynamics (MD) simulations and enhanced sampling free energy methods play an important role in elucidating the atomistic details of ion transport and the critical ion-ion and ion-polymer interactions in SPEs. In the present thesis, we have employed molecular dynamics (MD) simulation, enhanced sampling methods (for example, the adaptive biasing force (ABF) method, well-tempered metadynam- ics (WTmetaD) simulation, and nudged elastic band (NEB) method) and normal mode analy- sis (NMA) to investigate the structure, dynamics and energetics of crystalline and amorphous phases of poly(ethylene oxide) (PEO) and PEO-based SPEs. Thermal phase behavior of crystalline poly(ethylene oxide) explored using MD simula- tions has shown the possibility of a premelting transition well below the equilibrium melting of the crystal. At temperatures just above the premelting transition temperature, molecules are able to rotate about their long axes, and diffuse along the chain-axis of the crystal. To under- stand the nature of coupling between polymer dynamics and ion transport in SPEs, all-atom molecular dynamics simulations of crystalline PEO3:NaI were carried out and polymer con- formational dynamics, energetics and migration pathways of ions were characterized . The results reveal that in crystalline PEO3:NaI both anions and cations follow helical pathways mirroring the helical symmetry of the polymer. The normal mode analysis (NMA) and the power spectral analysis of MD-derived velocity autocorrelation functions (VACF) have eluci- dated the important roles of backbone twisting of PEO helical chains about their long axes and softening of a few low-frequency collective modes in ion transport in SPEs. The calculated potentials of mean force for cations reveal the existence multiple isoenergetic coordination states separated by low energy barriers in the amorphous SPEs indicating that ion transport is facile and the mechanism of ion conduction is different in the amorphous SPEs than those in the crystalline SPEs. Full thesis: pdf Centre for Computational Natural Sciences and Bioinformatics |
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